The movie opens with that question. In response, Jayson Blair jokes. “This one again,” he mutters, rolling his eyes in mock consternation at the predictability of it.

But predictable as it is and as long as he’s had to ponder it, Blair still ends up punting. “I don’t have a good answer for the question,” he acknowledges.

“A Fragile Trust: Plagiarism, Power and Jayson Blair at The New York Times” — it premieres this week on PBS — reintroduces us to the central figure in one of the great media scandals of all time, the one-time wunderkind who lied and plagiarized his way through a career on what is arguably the greatest stage in American journalism, The New York Times. He claimed to have been places he had not gone, to have interviewed people he had not met, to have witnessed scenes he had not seen. He stole the work of other reporters and passed it off as his own. And he did this dozens of times.

So why?

At the time — spring, 2003 — some pundits claimed the scandal reflected an unseemly obsession with diversity, the supposed inability of white editors strait-jacketed by political correctness to confront a black reporter’s incompetence. Blair worked the other side of the street, claiming in a bizarre New York Observer interview that racism was a factor in his betrayal of the most basic ethical tenets of his profession.

Neither claim made much sense. They seem preposterous now.

In her search for the answer to the question, filmmaker Samantha Grant follows Blair — now pushing 40 — through the life he has made since his old life crumbled. She takes us to the nondescript office where he works as, of all things, a life coach and to a university where he lectures skeptical students on — wait for it — journalistic ethics. We learn that he was a brown noser, an abuser of alcohol and drugs and had bipolar disorder.

All useful details, but none answers that question. Why?

Then, at one point, Blair muses on the fact that many of his earliest mistakes were caught only because he reported them — which is the way things are routinely done in a business that is, after all, based largely on trust and trustworthiness. But that wasn’t the takeaway for Blair. He says:

“(O)nce you cross that barrier where you know the chances are you won’t be caught, it becomes very hard to discipline yourself.”

His tone presumes anyone would do the same had they realized what he did. But is that really the way it is for most of us? If you had no fear of being caught, would you betray professional ethics? Break a window? Rob a store? Isn’t the answer, for most of us, no?

Maybe what happened happened not because of skin color, substance abuse, mental illness or social immaturity but because Blair is defective, afflicted by a sociopathic inability to comprehend ordinary human mores. Maybe he did it because he could.

That’s the answer to the question. Or at least, it is as close as we are ever likely to come.